Coding Horror - The Rule Of Three

There are two "rules of three" in [software] reuse:

1. It is three times as difficult to build reusable components as single use components
2. and a reusable component should be tried out in three different applications before it will be sufficiently general to accept into a reuse library.

From http://blog.codinghorror.com/rule-of-three/

Having been looking after a lot of one-off projects that we hope will become reusable units, and dealing with the whit-ifs and second guessing involved in attempting to design for that, this article has been particularly relevant to me